"An estimated 11 million unauthorized aliens reside in the United States, and this population is estimated to increase by 500,000 annually. Each year, approximately 1 million aliens are apprehended trying to enter the United States illegally. Although most of these aliens enter the United States for economic opportunities and family reunification, or to avoid civil strife and political unrest, some are criminals, and some may be terrorists. All are violating the United States' immigration laws. Immigration enforcement is the regulation of those who violate provisions of the Immigration and Nationality Act (INA). This includes violations of the INA's civil provisions (e.g., violate the conditions of their admittance), as well as U.S. citizens or aliens who violate the criminal provisions (e.g., marriage fraud or alien smuggling). Many divergent tasks are incorporated under the banner of immigration enforcement. These include removing aliens who should not be in the United States, investigating alien smuggling and trafficking, patrolling between and at ports of entry, combating document and benefit fraud, and enforcing the prohibitions against employers hiring aliens without work authorization."
CRS Report for Congress, RL33351